Add a way to create an unbound group element var handle

Consider this layouts:

static final GroupLayout POINT = MemoryLayout.structLayout(
            ValueLayout.JAVA_INT.withName("x"),
            ValueLayout.JAVA_INT.withName("y")
);

static final GroupLayout LINE = MemoryLayout.structLayout(
            POINT.withName("p1"),
            POINT.withName("p2")
);

There should be some way to obtain a var handle which can dereference a Point::x in both p1, p2, using an index that is provided dynamically.

A possible way to do this, would be to add a new PathElement factory:

PathElement.groupElements(String... elems)

Where the invariants are:

* all the names in elems must be defined in the layout
* all the layouts corresponding to elems must equals (modulo outer layout name)